Projectors ordinarily are designed to produce a rectangular image when projecting onto a screen or other projection surface from an angle that is perpendicular to or aligned with the normal of the projection surface. Image distortion occurs when a projector is aligned at a different, oblique angle to the projection surface, or when the projection surface itself is angled, curved, or in some way irregular with respect to the projector. The keystone effect is one of the most common forms of distortion which results in an image that is trapezoidal rather than rectangular.
In order to address the keystoning problem, projectors typically utilize either optical or digital approaches to correct image distortion. Optical keystone correction involves a physical adjustment to the projector lens so that the projector projects at a different angle than it would be for a normal projection angle on a flat projection surface. Optical keystone correction generally is more effective if the projector is sufficiently far from the projection surface. Digital keystone correction involves applying correction within digital video processing by converting and prescaling the image before the image is projected. One disadvantage of prescaling the video image is that the number of individual pixels in the image is reduced, thereby lowering the display resolution and degrading the projected image quality.
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